1. Field of the Invention
The present invention relates to a lithographic apparatus, a device manufacturing method and a radiation system. The present invention relates to a lithographic apparatus designed for use with radiation having a wavelength in the extreme ultraviolet (EUV) range.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In this case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC. This is done using a projection system that is between the reticle and the substrate and is provided to image an irradiated portion of the reticle onto a target portion of a substrate. The projection system includes components to direct, shape and/or control a beam of radiation. The pattern can be imaged onto the target portion (e.g. including part of one, or several, dies) on a substrate, for example a silicon wafer, that has a layer of radiation-sensitive material, such as resist. In general, a single substrate contains a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction, usually referred to as the “scanning” direction, while synchronously scanning the substrate parallel or anti-parallel to this direction.
An important aspect in lithography is the size of features of the pattern applied to the substrate. It is desirable to produce apparatus capable of resolving features as small and close together as possible. A number of parameters affect the available resolution of features. One of these is the wavelength of the radiation used to expose the pattern. Using radiation with an EUV wavelength between 5 and 20 nm, and typically 13.5 nm, it is anticipated that it will be possible to manufacture feature sizes down to 32 nm.
Various EUV sources are known, for example some plasma-based radiation sources emit radiation in this wavelength range. These sources are volume radiators. By this it is meant that the radiator is (virtually) transparent for the radiation it emits and so the radiation produced inside the volume propagates freely towards the surface of the volume, and passes this surface without interacting with the radiating species. Plasma sources can be stimulated either by using suitable laser radiation or by using an electrical discharge. These sources come in many different forms, and are well known in the art. Various examples are described in WO 01/99143.
While using wavelengths in the EUV range allows for the fabrication of very small features, it can cause practical problems. Radiation at this wavelength is absorbed in all materials and is therefore not suitable for use with refractive optics. The optics in a projection system for use with EUV lithography must therefore be based on mirrors, which can only operate in an ultra high vacuum (UHV) environment. A further problem is that the conversion efficiency, i.e. the ratio of power out at the required wavelength to power in, for discharge sources is very low, which means that the radiation power output is correspondingly low.